user control raising events

My suggest is:
Solution 1: create 1 static variable in user control 2

Solution 2:
In aspx page, add 1st user control, 2st control will be add dynamically in code behind

Create an property in 2st user control, such as;
public string LabelInControl2 { get; set;}

At 1st user control, gridview selecting item event, you will add 2st user control depend on which item is selected, code example:

// create dynamic user control
string path= "your_2st_user_control_url.ascx";
UserControl2 uc2 = (UserControl2)Page.LoadControl(path);

// set control property
uc2.LabelInControl2 = gridviewitem

// load control in main aspx
this.Page.Controls.Add(uc2)